In recent years, global warming and energy problems have become increasingly serious. If a great amount of waste heat generated in plants or vehicles, or solar energy can be recovered with high efficiency, it may be a last resort for solving the global warming and the energy problems. In order to recover the energy to convert the recovered energy to power, researches on thermoacoustic engines have been actively conducted.
A thermoacoustic engine uses self-excited oscillation generated in a pipe. Shortly, a bundle of narrow channels (hereinafter, referred to as a regenerator) is installed in the pipe. When a temperature ratio at both ends of the regenerator is set at or above a certain critical value, a fluid in the pipe causes the self-excited oscillation. This effect can be thermodynamically regarded as a motor without moving parts and the thermoacoustic engine is realized by using the effect (for example, see Patent Documents 1 and 2). Since the thermoacoustic engine is an external engine which is driven in the Stirling cycle, there is a possibility that work can be extracted with high-efficiency from any heat sources such as sunlight and industrial waste heat. Also, since the thermoacoustic engine has a simple structure which exchanges heat by use of sound waves, any moving parts such as pistons and turbines are not necessary at all, which is different from the usual Stirling engine. Therefore, advantages of inexpensiveness, a long service life and maintenance-free can be obtained.
A structure of a typical thermoacoustic engine (for example, see Non-Patent Document 1) is illustrated in FIGS. 10A and 10B, which has been researched for aiming at practical use in recent years. A thermoacoustic generator 500 illustrated in FIG. 10A is provided with a loop pipe 100 and a resonance pipe 111. In the loop pipe 100, a regenerator 210, a heater 220 and a cooler 230 forming a motor 200 are provided. A generator (linear generator) 300 is provided at one end of the resonance pipe 111. In the thermoacoustic generator 500, when a temperature gradient is given to the regenerator 210, self-excited oscillation as sound waves (that is, thermoacoustic self-excited oscillation) is excited, and the linear generator 300 converts oscillation energy (that is, acoustic energy) E of the sound waves to electric energy. The thermoacoustic generator 500 is intended for use as a solar generator having high-efficiency over waste heat utilization generators and solar panels.
While, researches on coolers, refrigerators and a thermoacoustic refrigerator 600 (for example, see Non-Patent Document 2) illustrated in FIG. 10(b) as device for generating cryogenic temperature have also been actively conducted. The thermoacoustic refrigerator 600 has two loop pipes 100, 120 and a resonance pipe 111. In the loop pipe 100, a regenerator 210, a heater 220 and a cooler 230 forming a motor 200 are provided. In the loop pipe 120, a refrigerating regenerator 410, a cold air discharger 420 and a refrigerating cooler 430 forming a refrigerator 400 are provided. In the thermoacoustic refrigerator 600, when a temperature gradient is given to the regenerator 210 installed in one loop pipe 100, the self-exerted oscillation is excited. Acoustic energy E by the self-excited oscillation is transmitted to the other loop pipe 120 via the resonance pipe 111. The refrigerating regenerator 410 works for refrigeration by executing the reverse Stirling cycle. The thermoacoustic refrigerator 600 which generates a low temperature with the self-excited oscillation as such in-pipe sound waves has a potential over pulse pipe refrigerators.
Many companies conduct researches on the typical thermoacoustic engines above in view of neat recovery and next-generation energy utilization. However, since a full-scale study has been conducted in the 21st century for this new field, a fundamental technology has not yet been established.
An operation temperature of the thermoacoustic engines is around 500 degrees C. for general use (see Non-Patent Document 3). This temperature is much higher than a waste heat temperature (around 100 degrees C. to 300 degrees C.) discharged from actual cars or factories. Therefore, as an attempt to lower the operation temperature of the thermoacoustic engines, in recent years, a “multistage thermoacoustic engine” by arranging regenerators in series in a multistage is proposed, by which power amplification of working flow W is realized in each regenerator (see Non-Patent Document 4).